Powder coating compositions

ABSTRACT

A powder coating compositions incorporates a wax in post-blended form, the composition incorporating the wax and the same composition without the wax being separated, preferably widely separated, in a triboelectric reference series indicative of the extent to which powder coating compositions may be distinguishable from one another when electrically charged. The powder coating compositions incorporating a wax in post-blended form is especially suitable for coating articles having recessed portions. A quantitative basis is given for determining separation in the triboelectric reference series, and preferred minimum separation criteria are given. The effects obtainable using post-blended wax may be enhanced by the use, as further post-blended additives, of a combination of aluminium oxide and aluminium hydroxide.

[0001] This invention relates to powder coating compositions and totheir use in coating substrates, especially substrates of complicatedshape, with particular reference to articles having recessed portions.

[0002] Powder coating compositions generally comprise a solidfilm-forming resin binder, usually with one or more colouring agentssuch as pigments, and optionally also contain one or more performanceadditives. They are usually thermosetting, incorporating, for example, afilm-forming polymer and a corresponding curing agent (which may itselfbe another film-forming polymer), but thermoplastic systems (based, forexample, on polyamides) can in principle be used instead. Powder coatingcompositions are generally prepared by intimately mixing the ingredients(including colouring agents and performance additives) for example in anextruder, at a temperature above the softening point of the film-formingpolymer(s) but below a temperature at which significant pre-reactionwould occur. The extrudate is usually rolled into a flat sheet andcomminuted, for example by grinding, to the desired particle size. Otherhomogenisation processes also come into consideration, includingnon-extruder-based processes such as, for example, processes involvingmixing using supercritical fluids, especially carbon dioxide.

[0003] Powder coating compositions are generally applied by anelectrostatic spray process in which the powder coating particles areelectrostatically charged by the spray gun and the substrate (normallymetallic) is earthed. The charge on the powder coating particles isnormally applied by interaction of the particles with ionised air(corona charging) or by friction (tribostatic or “tribo” charging). Thecharged particles are transported in air towards the substrate and theirfinal deposition is influenced inter alia by the electric field linesthat are generated between the spray gun and the workpiece. Adisadvantage of this process is that there are difficulties in coatingarticles having complicated shapes, and especially articles havingrecessed portions, as a result of restricted access of the electricfield lines into recessed locations (the Faraday cage effect),especially in the case of the relatively strong electric fieldsgenerated in the corona-charging process. The Faraday cage effect ismuch less evident in the case of tribostatic charging processes, butthose processes have other drawbacks.

[0004] As an alternative to electrostatic spray processes, powdercoating compositions may be applied by fluidised-bed processes, in whichthe substrate workpiece is preheated (typically to 200° C.-400° C.) anddipped into a fluidised bed of the powder coating composition. Thepowder particles that come into contact with the preheated surface meltand adhere to the workpiece. In the case of thermosetting powder coatingcompositions, the initially-coated workpiece may be subjected to furtherheating to complete the curing of the applied coating. Such post-heatingmay not be necessary in the case of thermoplastic powder coatingcompositions.

[0005] Fluidised-bed processes eliminate the Faraday cage effect,thereby enabling recessed portions in the substrate workpiece to becoated, and are attractive in other respects, but have the well-knowndisadvantage that the applied coatings are substantially thicker thanthose obtainable by electrostatic coating processes.

[0006] Another alternative application technique for powder coatingcompositions is the so-called electrostatic fluidised-bed process, inwhich the fluidising air is ionised by means of charging electrodesarranged in the fluidising chamber or, more usually, in the plenumchamber below the porous air-distribution membrane. The ionised aircharges the powder particles, which acquire an overall upwards motion asa result of electrostatic repulsion of identically charged particles.The effect is that a cloud of charged powder particles is formed abovethe surface of the fluidised bed. The substrate workpiece (earthed) isintroduced into the cloud and powder particles are deposited on thesubstrate surface by electrostatic attraction. No preheating of thesubstrate workpiece is required.

[0007] The electrostatic fluidised-bed process is especially suitablefor coating small articles, because the rate of deposition of the powderparticles becomes less as the article is moved away from the surface ofthe charged bed. Also, as in the case of the traditional fluidised-bedprocess, the powder is confined to an enclosure and there is no need toprovide equipment for recycling and reblending the overspray that is notdeposited on the substrate. As in the case of the corona-chargingelectrostatic process, however, there is a strong electric field betweenthe charging electrodes and the substrate workpiece and, as a result,the Faraday cage effect operates to a certain extent and leads to poordeposition of powder particles into recessed locations on the substrate.

[0008] WO 99/30838 proposes a process which comprises the steps ofestablishing a fluidised bed of a powder coating composition, immersingthe substrate wholly or partially within the said fluidised bed,applying a voltage to the substrate for at least part of the period ofimmersion, whereby particles of the powder coating composition arecharged substantially by friction alone and adhere to the substrate,withdrawing the substrate from the fluidised bed and forming theadherent particles into a continuous coating over at least part of thesubstrate.

[0009] As compared with processes in which a substantial electric fieldis generated between charging electrodes and the substrate workpiece,the process of WO 99/30838, which is conducted without ionisation orcorona effects in the fluidised bed, offers the possibility of achievinggood coating of substrate areas which are rendered inaccessible by theFaraday cage effect.

[0010] The present invention provides a powder coating composition whichincorporates a wax in a post-blended form.

[0011] The term “post-blended” means that the wax material has beenincorporated after the extrusion or other homogenisation process (forconvenience referred to hereinafter simply as “extrusion”).

[0012] The use of post-blended wax in accordance with the inventionoffers the possibility of achieving improved Faraday cage penetration inthe coating of substrates and, as a result, more uniform coating ofsubstrates having recessed areas or other locations rendered difficultlyaccessible by the Faraday cage effect, for example, the internal comerregions of microwave ovens. In particular, the invention enables thedesired minimum coating thickness to be achieved on such regions withouthaving to apply excess material to other more easily accessible areas ofthe substrate. Substantial savings of powder coating material arepossible.

[0013] It will be understood that the use of post-blended wax inaccordance with the invention is clearly distinct from prior proposalsto incorporate wax for different purposes before or during extrusion.Such proposals can, however, be combined with the practice of thepresent invention.

[0014] The advantages of the invention are best seen in coronaapplication processes, but other application processes may in principlebe used instead, although the effect of the invention will generallythen be less pronounced.

[0015] The invention further provides a process for forming a coating ona substrate, in which a composition according to the invention isapplied to the substrate by a powder coating process, preferably acorona application process, resulting in particles of the compositionadhering to the substrate, and forming the particles into a continuouscoating.

[0016] The substrate is advantageously an article having recessedportions subject to the Faraday cage effect, and for an article havingmultiple faces the ratio of the minimum to maximum coating thickness isadvantageously at least 40%, preferably at least 50%.

[0017] The invention also provides the use of a powder coatingcomposition of the invention in coating an article having recessedportions which may be, for example, the interior of a refrigerator ormicrowave oven, an alloy wheel, an architectural extrusion or a radiatorfin.

[0018] The wax in a powder coating composition of the invention isadvantageously a synthetic wax, preferably a polyethylene (PE) orpolytetrafluoroethylene (PTFE) wax, a PE wax modified with PTFE orpolyamide, or a polyamide wax. In principle, however, other waxmaterials may be used instead, for example:

[0019] i) Natural animal waxes (for example, beeswax, lanolin);

[0020] ii) Natural vegetable waxes (for example, carnauba wax); or

[0021] iii) Natural petroleum or other mineral waxes (for example,paraffin wax, microcrystalline wax); or

[0022] iv) any of classes i)-iii) modified by PTFE or polyamide.

[0023] An important group of waxes for use in accordance with theinvention comprises esters of long-chain aliphatic alcohols (typicallyC₁₆ and above) with long-chain fatty acids (typically C₁₆ and above).Such esters and acids are preferably straight-chain compounds, and maybe saturated or unsaturated. Examples of acids which may be used includestearic acid, palmitic acid and oleic acid and mixtures of two or morethereof.

[0024] Waxes derived from long-chain aliphatic compounds as describedabove may include hydrocarbons.

[0025] In addition to esters of the long-chain acids as described abovethere may be mentioned salts such as, for example, aluminium stearate.

[0026] Preferred wax materials for use in accordance with the inventionare materials which have good compatibility with the polymercomponent(s) of the powder coating composition, that is to say,materials which can be mixed homogeneously with the polymers withoutsignificant phase separation. It will be found that some wax materials(for example, halogenated waxes) are in general not compatible in thissense with the powder coating polymer(s). The use of such materialswould be expected to give rise to defects in the surface appearance ofthe finished applied coating, and is accordingly not recommended.

[0027] Particular examples of suitable waxes include the followingmanufactured by Lubrizol: LANCO WAX A. 1601 (a fatty acid amide wax),LANCO WAX HM. 1666 (an amide wax) and LANCO WAX TF 1725 (a PTFE-modifiedpolyethylene wax).

[0028] The amount of wax may be in the range 0.03-2%, but mention may bemade of amounts in the range of from 0.03 to 0.8% by weight and 0.03 to0.5% by weight. In addition, care is necessary to ensure that the powdercoating composition does not become too sticky, and it may also be foundthat the penetration-enhancing effect of post-blended wax will diminish,with increasing wax addition, after a maximum value has been reached.The preferred maximum wax content will in general be 0.3 or 0.2%, moreespecially not exceeding 0.1%, all percentages being by weight and beingbased on the weight of the composition without the wax. Particularmention may be made of amounts in the range of from 0.05 to 0.1% byweight, especially 0.07 to 0.1%.

[0029] In general, the T_(g) of the wax should be above that of theremainder of the powder coating composition. This serves to reduce thetendency of the composition to become sticky as a result ofincorporation of the wax. Preferably, the T_(g) of the wax is in therange of from 100° to 140° C.

[0030] In principle, more than one wax may be used as post-blendedadditive in accordance with the invention. In general, however, the useof a plurality of waxes will militate against the achievement of optimumresults. If more than one wax is to be used, it is considered preferableto divide the base composition into a corresponding number of portions,post-blend a different wax with each portion and then mix the resultingpowders together. Incorporation of two or more waxes in the samepost-blending operation is not recommended.

[0031] Post-blending of the wax may be achieved, for example, by any ofthe following dry-blending methods:

[0032] a) tumbling the wax into the chip before milling;

[0033] b) injection at the mill, with the chip and wax fed into the millsimultaneously;

[0034] c) introduction at the stage of sieving after milling;

[0035] d) post-production blending in a “tumbler” or other suitablemixing device; or

[0036] e) introduction into a fluidised-bed powder reservoir supplyingan electrostatic powder application gun.

[0037] In the case of method a) or b), the particle size of the wax ispreferably less than that of the chip, and advantageously <50 microns.In the case of method c), d) or e), the particle size of the wax ispreferably less than that of the powder coating composition, preferably<30 microns, more especially <15 microns, for example <10 microns.

[0038] The effects obtainable by the use of post-blended wax inaccordance with the invention may be enhanced by the use, as furtherpost-blended additives, of a combination of aluminium oxide andaluminium hydroxide, typically in proportions in the range of from 1:99to 99:1 by weight, advantageously from 10:90 to 90:10, preferably from30:70 to 70:30, for example, from 45:55 to 55:45. The combination ofaluminium oxide and aluminium hydroxide is disclosed in WO 94/11446 as afluidity-assisting post-blended additive. Other combinations of theinorganic materials disclosed in WO 94/11446 may in principle also beused in the practice of the present invention.

[0039] Such further post-blended additives may be incorporated with thecomposition simultaneously with the wax or separately from it, and maybe incorporated by any of the post-blending techniques described inrelation to the wax. Although any such additive or mixed sub-combinationof additives may in principle be incorporated separately in the powdercoating composition, pre-mixing of additives (other than the wax) isgenerally preferred.

[0040] Combinations of aluminium oxide and aluminium hydroxide (andsimilar additives) are advantageously used in amounts in the range offrom 0.25 to 0.75% by weight, preferably 0.45 to 0.55%, based on theweight of the composition without the additives. Amounts up to 1% or 2%by weight may be used, but problems can arise if too much is used, forexample, bit formation and decreased transfer efficiency.

[0041] Whilst the post-blended wax may in principle be in the form ofwax deposited on a carrier material (such as, for example, silica), theuse of such inhomogeneous materials is in general not recommended in thepractice of the present invention.

[0042] The particle size distribution of the powder coating compositionmay be in the range of from 0 to 150 microns, generally up to 120microns, with a mean particle size in the range of from 15 to 75microns, preferably at least 20 or 25 microns, advantageously notexceeding 50 microns, more especially 20 to 45 microns. Although theinvention can in principle offer advantages over the whole range ofparticle size distributions, it has been found that the benefits interms of Faraday cage penetration tend to be less pronounced inrelatively fine particle size distributions.

[0043] A powder coating composition according to the invention maycontain a single film-forming powder component comprising one or morefilm-forming resins or may comprise a mixture of two or more suchcomponents.

[0044] The film-forming resin (polymer) acts as a binder, having thecapability of wetting pigments and providing cohesive strength betweenpigment particles and of wetting or binding to the substrate, and meltsand flows in the curing/stoving process after application to thesubstrate to form a homogeneous film.

[0045] The or each powder coating component of a composition of theinvention will in general be a thermosetting system, althoughthermoplastic systems (based, for example, on polyamides) can inprinciple be used instead.

[0046] When a thermosetting resin is used, the solid polymeric bindersystem generally includes a solid curing agent for the thermosettingresin; alternatively two co-reactive film-forming thermosetting resinsmay be used.

[0047] The film-forming polymer used in the manufacture of the or eachcomponent of a thermosetting powder coating composition according to theinvention may be one or more selected from carboxy-functional polyesterresins, hydroxy-functional polyester resins, epoxy resins, andfunctional acrylic resins.

[0048] A powder coating component of the composition can, for example,be based on a solid polymeric binder system comprising acarboxy-functional polyester film-forming resin used with a polyepoxidecuring agent. Such carboxy-functional polyester systems are currentlythe most widely used powder coatings materials. The polyester generallyhas an acid value in the range 10-100, a number average molecular weightMn of 1,500 to 10,000 and a glass transition temperature Tg of from 30°C. to 85° C., preferably at least 40° C. The poly-epoxide can, forexample, be a low molecular weight epoxy compound such as triglycidylisocyanurate (TGIC), a compound such as diglycidyl terephthalatecondensed glycidyl ether of bisphenol A or a light-stable epoxy resin.Such a carboxy-functional polyester film-forming resin can alternativelybe used with a bis(beta-hydroxyalkylamide) curing agent such astetrakis(2-hydroxyethyl) adiparnide.

[0049] Alternatively, a hydroxy-functional polyester can be used with ablocked isocyanate-functional curing agent or an amine-formaldehydecondensate such as, for example, a melamine resin, a urea-formaldehyeresin, or a glycol ural formaldehye resin, for example the material“Powderlink 1174” supplied by the Cyanamid Company, or hexahydroxymethylmelamine. A blocked isocyanate curing agent for a hydroxy-functionalpolyester may, for example, be internally blocked, such as the uretdionetype, or may be of the caprolactam-blocked type, for example isophoronediisocyanate.

[0050] As a further possibility, an epoxy resin can be used with anamine-functional curing agent such as, for example, dicyandiamide.Instead of an amine-functional curing agent for an epoxy resin, aphenolic material may be used, preferably a material formed by reactionof epichlorohydrin with an excess of bisphenol A (that is to say, apolyphenol made by adducting bisphenol A and an epoxy resin). Afunctional acrylic resin, for example a carboxy-, hydroxy- orepoxy-functional resin can be used with an appropriate curing agent.

[0051] Mixtures of film-forming polymers can be used, for example acarboxy-functional polyester can be used with a carboxy-functionalacrylic resin and a curing agent such as a bis(beta-hydroxyalkylamide)which serves to cure both polymers. As further possibilities, for mixedbinder systems, a carboxy-, hydroxy- or epoxy-functional acrylic resinmay be used with an epoxy resin or a polyester resin (carboxy- orhydroxy-functional). Such resin combinations may be selected so as to beco-curing, for example a carboxy-functional acrylic resin co-cured withan epoxy resin, or a carboxy-functional polyester co-cured with aglycidyl-functional acrylic resin. More usually, however, such mixedbinder systems are formulated so as to be cured with a single curingagent (for example, use of a blocked isocyanate to cure ahydroxy-functional acrylic resin and a hydroxy-functional polyester).Another preferred formulation involves the use of a different curingagent for each binder of a mixture of two polymeric binders (forexample, an amine-cured epoxy resin used in conjunction with a blockedisocyanate-cured hydroxy-functional acrylic resin).

[0052] Other film-forming polymers which may be mentioned includefunctional fluoropolymers, functional fluorochloropolymers andfunctional fluoroacrylic polymers, each of which may behydroxy-functional or carboxy-functional, and may be used as the solefilm-forming polymer or in conjunction with one or more functionalacrylic, polyester and/or epoxy resins, with appropriate curing agentsfor the functional polymers.

[0053] Other curing agents which may be mentioned include epoxy phenolnovolacs and epoxy cresol novolacs; isocyanate curing agents blockedwith oximes, such as isopherone diisocyanate blocked with methyl ethylketoxime, tetramethylene xylene diisocyanate blocked with acetone oxime,and Desmodur W (dicyclohexylmethane diisocyanate curing agent) blockedwith methyl ethyl ketoxime; light-stable epoxy resins such as “SantolinkLSE 120” supplied by Monsanto; and alicyclic poly-epoxides such as“EHPE-3150” supplied by Daicel.

[0054] A powder coating composition for use according to the inventionmay be free from added colouring agents, but usually contains one ormore such agents (pigments or dyes). Examples of pigments which can beused are inorganic pigments such as titanium dioxide, red and yellowiron oxides, chrome pigments and carbon black and organic pigments suchas, for example, phthalocyanine, azo, anthraquinone, thioindigo,isodibenzanthrone, triphendioxane and quinacridone pigments, vat dyepigments and lakes of acid, basic and mordant dyestuffs. Dyes can beused instead of or as well as pigments.

[0055] The composition of the invention may also include one or moreextenders or fillers, which may be used inter alia to assist opacity,whilst minimising costs, or more generally as a diluent.

[0056] The following ranges should be mentioned for the totalpigment/filler/extender content of a powder coating compositionaccording to the invention (disregarding post-blend additives):

[0057] 0% to 55% by weight,

[0058] 0% to 50% by weight,

[0059] 10% to 50% by weight,

[0060] 0% to 45% by weight, and

[0061] 25% to 45% by weight

[0062] Of the total pigment/filler/extender content, the pigment contentwill generally be ≦40% by weight of the total composition (disregardingpost-blend additives) but proportions up to 45% or even 50% by weightmay also be used. Usually a pigment content of 25-35% is used, althoughin the case of dark colours opacity can be obtained with <10% by weightof pigment.

[0063] The composition of the invention may also include one or moreperformance additives, for example, a flow-promoting agent, aplasticiser, a stabiliser against UV degradation, or an anti-gassingagent, such as benzoin, or two or more such additives may be used. Thefollowing ranges should be mentioned for the total performance additivecontent of a powder coating composition according to the invention(disregarding post-blend additives):

[0064] 0% to 5% by weight,

[0065] 0% to 3% by weight, and

[0066] 1% to 2% by weight.

[0067] In general, colouring agents, fillers/extenders and performanceadditives as described above will not be incorporated by post-blending,but will be incorporated before and/or during the extrusion or otherhomogenisation process.

[0068] After application of the powder coating composition to asubstrate, conversion of the resulting adherent particles into acontinuous coating (including, where appropriate, curing of the appliedcomposition) may be effected by heat treatment and/or by radiant energy,notably infra-red, ultra-violet or electron beam radiation.

[0069] The powder is usually cured on the substrate by the applicationof heat (the process of stoving); the powder particles melt and flow anda film is formed. The curing times and temperatures are interdependentin accordance with the composition formulation that is used, and thefollowing typical ranges may be mentioned: Temperature/° C. Time 280 to100* 10 s to 40 min 250 to 150 15 s to 30 min 220 to 160 5 min to 20 min

[0070] The invention is applicable over a wide range of applied filmthicknesses, typically from thin films of, for example, 30 microns orless up to films of 50, 100, 150 or 200 microns. A typical minimum filmthickness is 5 microns.

[0071] As a generality, for any given powder coating composition, theextent of advantage gained by the use of post-blended wax in accordancewith the invention is dependent on the nature of the wax used. Morespecifically, it has been found in accordance with the invention thatthe results in terms of Faraday cage penetration can be enhanced byselecting the wax taking into consideration a measure of the tendency ofthe base composition to become positively or negatively charged in atribocharging environment.

[0072] In one approach, mixtures consisting of one part which is a basicpowder coating composition and another part which is the basic powdercoating composition treated with a wax are charged tribostatically andthe basic part is found to become charged predominantly in one sensewhile the wax-treated part is found to become charged predominantly inthe opposite sense, permitting the separation of the mixture into thebasic part and the wax-treated part by directing it at two oppositelycharged plates. It is found that some mixtures of basic-part andwax-treated-part powder coating compositions separate to a greaterextent than do others when directed at oppositely charged plates.

[0073] The fact that the basic-part and the wax-treated part of a powdercoating composition are found to become oppositely charged provides abasis for establishing a triboelectric series of the powder coatingcompositions including basic powder coating compositions with andwithout wax treatment. The basic powder coating compositions themselvesare known to be separable when mixed with one another and chargedtribostatically, one basic powder coating composition acquiring apositive charge while the other acquires a negative charge, as shown bya tendency to separate onto two oppositely charged plates. In theresulting triboelectric series, the relative positions of the basic andwax-treated powder coating compositions are such that each powdercoating composition takes on a negative charge in a charged mixture withthe powder coating composition positioned immediately above it and apositive charge in a charged mixture with the powder coating compositionpositioned immediately below it.

[0074] The fact that some charged mixtures separate to a greater extentthan do others leads to the expectation that basic and wax-treatedpowder coating compositions occupying widely separated positions in thetriboelectric series separate from each other to a greater extent thando basic and wax-treated powder coating compositions that occupyadjacent positions in the triboelectric series.

[0075] A procedure for establishing a triboelectric series for thepurposes of the present invention may include the following steps:

[0076] (i) selecting a plurality of powder coating compositions forinclusion in the triboelectric series,

[0077] (ii) selecting a first two of the powder coating compositions,

[0078] (iii) mixing the two selected powder coating compositions insubstantially equal amounts,

[0079] (iv) causing tribostatic charging of the mixture of powdercoating compositions by tribostatic interaction to establish equilibriumtribostatically charged conditions,

[0080] (v) directing the tribostatically charged mixture at twoelectrically charged plates of opposite polarities relative to eachother,

[0081] (vi) identifying which of the two powders adheres to theelectrically positive plate,

[0082] (vii) so allocating positions to the two powder coatingcompositions in the triboelectric series that the the powder coatingcomposition which adheres to the positive plate occupies a positionimmediately below the position of the powder coating composition whichadheres to the negative plate,

[0083] (viii) repeating the steps (ii) to (vii) until all of the powdercoating compositions have been tested in pairs and allocated positionsin the triboelectric series.

[0084] The steps (iv) and (v) above may be combined by ejecting themixed powder coating compositions from a powder application gun suppliedfrom a fluidised-bed hopper.

[0085] In a procedure which maintains the separation of the steps (iv)and (v) above, the step (iv) comprises placing two powders in a glassjar, shaking the glass jar for a set period, for example, about twominutes then allowing a 30 second relaxation time.

[0086] In a preferred procedure again maintaining the separation of thesteps (iv) and (v) above, the step (iv) comprises fluidising the mixtureand allowing it to develop its equilibrium natural tribostatic charge.

[0087] When the above procedure is performed on a plurality of colouredbasic powder coating compositions visual identification of the basicpowder coating compositions is permitted. Black powder coatingcompositions and white powder coating compositions may, of course, beincluded.

[0088] An adequate number of basic powder coating compositions forestablishing a triboelectric series is seven and more than sevenprovides a more comprehensive triboelectric series. A minimum number ofbasic powder coating compositions for the triboelectric series is of theorder of five. Specific materials may be included in the series in orderto indicate reference positions although such materials are notnecessarily included in powder coating compositions. Suitable referencematerials are PTFE (polytetrafluoroethylene) occupying the lowestpossible position and polyamide occupying the highest possible positionin the triboelectric series.

[0089] The triboelectric series should include at least one pair ofbasic powder coating compositions which, when subjected to the abovemixing, charging and separation procedure, separate between the chargedplates to the extent that substantially all of one basic powder adheresto the positive plate and substantially all of the other basic powderadheres to the negative plate. Two such basic powder coatingcompositions fully satisfy the requirement for powder coatingcompositions that are well-separated in terms of triboelectricperformance. Analogously, the triboelectric series will include basicpowder coating compositions which, when subjected to the above mixing,charging and separation procedure, separate little or not at all betweenthe charged plates. Two powder coating compositions that make upmixtures which separate little or not at all fail to meet therequirement for powder coating compositions that are well-separated interms of triboelectric performance.

[0090] Where two differently coloured powder coating compositions aresubjected to the above mixing, charging and separation procedure and thetwo powder coating compositions fully satisfy the requirement for powdercoating compositions that are well-separated in terms of triboelectricperformance, the result is that the colour of the powder coatingcomposition adhering to the positive plate is substantially the samecolour as one powder coating composition, the colour of the powdercoating composition adhering to the negative plate being substantiallythe same as the colour of the other powder coating composition. Itfollows that a subjective quantitative assessment of the triboelectricperformance of two differently coloured powders is possible by visualinspection of the colours of the powder coating compositions on thepositive and negative plates relative to the respective colours of thepowder coating compositions before they are mixed.

[0091] An objective quantitative assessment of of the triboelectricperformance of two differently coloured powders is made with theassistance of a close tolerance reference colour spectrophotometercapable of operating in accordance with the CIE L*a*b*₁₉₇₆ system forassessing differences between colour samples. CIE is an abbreviation ofCommission International d'Eclairage.

[0092] A suitable spectrophotometer is a Spectraflash SF600 PLUS CTmanufactured by Datacolor International.

[0093] The CIE L*a*b*₁₉₇₆ system is a standard for defining colours interms of a three-dimensional coordinate system and, for rectangularcoordinates, a* is the x-coordinate variable, b* is the y-coordinatevariable and L* is the z-coordinate variable. The range of L* is 0 to100 and the ranges of a* and b* are both −100 to 100.

[0094] The following reference coordinates are included in the CIEL*a*b*₁₉₇₆ system:

[0095] Green: a*=−100, b*=0, L*=50

[0096] Red: a*=100, b*=0, L*=50

[0097] Blue: a*=0, b*=−100, L*=50

[0098] Yellow: a*=0, b*=100, L*=50

[0099] White: a*=0, b*=0, L*=100

[0100] Black: a*=0, b* =0, L*=0

[0101] The colour spectrophotometer operating in accordance with the CIEL*a*b*₁₉₇₆ system is capable of expressing the separation between twocolour pigments as ΔE, where ΔE²=ΔL*²+Δa*²+Δb*² where ΔL*, Δa* and Δb*are measured in the z, x and y directions, respectively. The magnitudeof ΔE is (ΔL*²+Δa*²+Δb*²)^(1/2).

[0102] Elementary electrostatics permits the separation of oppositelycharged particles by directing them towards oppositely charged plates.The negative particles are collected on the positive plate, and viceversa. Provided that there is some discernible difference between thetwo types of particle then the procedure permits the quantification ofthe degree of separation between two species in the mixture, by the useof differently coloured particles.

[0103] Established procedures for describing the charging behaviour ofpowder coatings use bulk measurements, which are relatively crude inassessing the charge characteristics of powders. By way of example,consider the following two cases: Case A Case B 2 particles of charge +32 particles of charge +8 2 particles of charge −2 2 particles of charge−7 Total bulk charge = +2 Total bulk charge = +2

[0104] A bulk charge measurement according to established procedureswould be incapable of distinguishing between these two cases. As far aswe are aware, there is no commercially available equipment forquantifying the charge distribution in powder coating compositions, soan indirect measurement of the charge behaviour must be made and that isachieved in accordance with the invention by the use of the parameter τas explained hereinafter. The degree of charge separation in case A issubstantially less than that in case B, and it has been found that theapplication of τ permits the selection of case B rather than case A asthe mixture capable of showing the higher separation.

[0105] Quantification is most readily achieved in respect of twocoloured powder coating compositions between which a significant ΔEexists. A value of ΔE (pure) between the pure powder coatingcompositions is first determined. The two powder coating compositionsare then mixed in equal weight proportions, caused to becometribostatically charged and the charged mixture sprayed through a powderdelivery gun at two oppositely charged plates, resulting in a degree ofseparation of the two powder coating compositions on to the two chargedplates according to the relative charges acquired by the two powdercoating compositions. The tribostatic charging, preferably, includesfluidising the mixture and allowing it to develop its equilibriumnatural tribostatic charge. After suitable treatment, for examplestoving, causing the powder coating compositions to become fixed to thetwo plates, a value ΔE (mixture) is determined between the powdercoating compositions on the two plates.

[0106] In accordance with the invention a parameter τ has been developedas a practical tool in the assessment of the triboelectric performanceof two differently coloured powders using the parameter ΔE. Theparameter τ is defined as τ=ΔE(mixture)/ΔE(pure). ΔE(pure) indicates avalue for ΔE between two pure powders. The determination of ΔE (mixture)comprises mixing the two powders in about equal weight proportions,causing the charging of the resulting mixture by tribostatic interactionto establish equilibrium tribostatically charged conditions, preferablyby fluidising, and causing the mixture to separate by spraying itthrough a powder delivery gun with no applied voltage at two oppositelycharged plates, ΔE(mixture) being the value of ΔE between the“separated” mixture distributed on the oppositely charged plates.

[0107] It has been found that the use of colour information permitspractical quantification of the extent to which tribostatically chargedpowder particles separate and that the results of colour measurementsare of practical value in the selection of highly-separating powdermixtures.

[0108] Preferably, a powder coating composition is characterised by atriboelectric interaction factor τ, between the compositionincorporating the wax and the same composition without the wax, of≧0.25, ≧0.3, ≧0.4, ≧0.5, ≧0.6, ≧0.7 or ≧0.8, the value of τ being givenby the relationship

τ=ΔE(composition mixture)/ΔE(pure compositions)

[0109] where

ΔE=(ΔL* ² +Δa* ² +Δb* ²)^(1/2)

[0110] with L*, a* and b* being respectively the z, y and x- coordinatevariables under the CIE L*a*b*₁₉₇₆ colour definition system,

[0111] ΔE (pure compositions) being determined by colourspectrophotometric measurement and ΔE (composition mixture) beingdetermined by mixing the two compositions in equal weight proportions,causing the charging of the resulting mixture by tribostatic interactionto establish equilibrium tribostatically charged conditions, directingthe charged mixture onto two oppositely charged plates, resulting in aseparation of the compositions on the two plates, and then determiningΔE, by colour spectrophotometric measurement, between the compositionsas applied to the two plates, one or both of the respective initial purecompositions being dyed where appropriate to provide an enhanced ΔEbetween them to facilitate the determination of ΔE (pure compositions)and ΔE (composition mixture).

[0112] The ratio τ=ΔE (mixture)/ΔE (pure) is attributed to the mixtureof the two powders. If, say, there has been total separation of thepowder coating mixture between the two plates, then ΔE (mixture) wouldbe the same as ΔE (pure) and the ratio τ would have a value of 1,possibly giving the same result as a subjective visual examination ofthe two plates. If, on the other hand, there has been no separation ofthe powder coating compositions between the two plates, the two plateswould be of substantially the same colour and ΔE (mixture) would besubstantially zero, leading to a ratio τ=0, which might be determined byvisual inspection of the two plates. The ratio τ can, of course, befound to attain any value between 0 and 1, both limits included,according to the value of ΔE (mixture) between the powder coatingcompositions adhering to the plates in relation to ΔE between the purepowder coating compositions.

[0113] A modified form of the above procedure is applied in the case oftwo coloured powder coating compositions between which there is not asignificant ΔE and, also, in the case of two white powders. Themodification involves the addition of a first dyestuff to one powderand, where appropriate to provide an enhanced ΔE, the addition of asecond dyestuff to the other powder coating composition, the addeddyestuffs being such as not to influence the relative charges acquiredby the powder coating compositions. The dyestuffs are so selected as tohave a significant ΔE and the remainder of the procedure set out aboveis followed in order to obtain ΔE for the mixture of the two powdercoating compositions. Following the addition of the dyestuffs, each dyedpowder coating composition should be checked in relation to thetriboelectric series in order to be sure that the addition of thedyestuff does not result in a change in the position of the powdercoating composition in the triboelectric series.

[0114] Dyestuffs may also be used for determining the triboelectricperformance of two white powder coating compositions following a check,as before, that the addition of the dyestuff does not cause a change inthe position of either powder coating composition in the triboelectricseries.

[0115] The value ΔE when used in the calculation of τ is considered togive accurate enough results for practical purposes although the use ofΔL*, Δa* and Δb* would be expected to provide more accuratedeterminations of τ.

[0116] It has been found that a value for ΔE of 2 is large enough togive satisfactory reproducible results in the determination of τ.

[0117] Values of τ greater than 0.25 have been noted to result inenhanced penetration of a mixture of powder coating compositionscompared with the penetration of the respective powders into recesses, avalue of τ greater than 0.5 is preferred and a value of T greater than0.6 is especially preferred. More generally, the value of τ may be ≧0.3,≧0.4 ≧0.5, ≧0.6, ≧0.7 or ≧0.8.

[0118] In the case of white powder coating compositions or colouredpowder coating compositions showing not much difference in ΔE, thetriboelectric performance may be quantified, alternatively oradditionally, by incorporating a small amount of two heavy metalcompounds into the respective powder coating compositions and measuringthe relative amounts of the heavy metal compounds in the powder coatingcompositions after mixture and separation on to oppositely chargedplates. The measurement would be by means of X-ray fluorescencespectroscopy or X-ray mass analysis using a scanning electronmicroscope.

[0119] For a mixture of a white basic powder coating composition and thewhite powder coating composition treated with a wax, a dyestuff is addedto the wax-coated part, say, before the two parts are mixed, as a meansof making it possible to determine T for the mixed basic-part andwax-treated part powder coated composition. A red dyestuff is chosen,added to a body of the basic powder coating composition and thebasic-without-dyestuff and basic-with-dyestuff powder coatingcompositions compared in accordance with the CIE L*a*b*₁₉₇₆ system todetermine ΔE(pure). Since red has coordinates of L*=50, a*=100, b*=0 inthe CIE L*a*b*, ₁₉₇₆ system, ΔE (pure)=Δa*(pure) provided Δb and ΔL arezero. An amount of the basic-with-dyestuff powder coating composition isthen treated with a specific amount of a selected wax, thebasic-with-dyestuff-with-wax and the basic powder coating compositionsare mixed together, caused to become tribostatically charged, separatedon to positive and negative plates and Δa*(mixture) is measured to giveτ(mixture)=Δa*(mixture)/Δa*(pure).

[0120] The red dyestuff may be substituted by a green dyestuff, in whichcase, for the above procedure, τ=Δa* (mixture)/Δa* (pure), since greenhas coordinates L*=50, a*=−100, b*=0 and again ΔE=Δa* provided Δb and ΔLare zero.

[0121] Dyestuffs may also be used with coloured powder coatingcompositions in order to determine τ for basic-part and wax-treated partmixtures of those powder coating compositions.

[0122] The proportion of the dyestuff needed, that is, the proportionneeded to achieve ΔE≧2, will in general be ≦0.4% by weight, althoughusually a lower proportion will suffice, say, of the order of 0.1%.

[0123] Having established a triboelectric series as described above, acorresponding determination is then made of the position in the seriesof the powder coating composition actually to be used for a givenapplication ( which may be a white or a coloured powder), hereinafterthe “end user powder.”

[0124] Advantageously, in the practice of the present invention, a waxis selected on the basis of the information provided by thetriboelectric series to provide basic end user and wax-treated end userpowder coating compositions which are separated in the triboelectricseries (in either the positive or negative direction) and, preferably,the basic end user and the wax-treated end user powder coatingcompositions are widely separated in the triboelectric series.

[0125] Preferably, the separation between the basic end user andwax-treated end user powder coating compositions as assessed by theabove method using the CIE L*a*b*₁₉₇₆ system gives a τ of more than 0.5and, preferably, more than 0.6.

[0126] The position of any given powder coating composition in thetriboelectric series may in principle be influenced by a number ofvariables, including:

[0127] (a) the nature and amount of any colouring agent (pigment ordye);

[0128] (b) the nature and amount of any filler/extender;

[0129] (c) the nature and amount of any post-blended additive;

[0130] (d) the use of a tribo-enhancing additive known from conventionaltribostatic application to enhance the tribostatic performance such as,for example, an amino alcohol or a tertiary amine or other suitablepre-extrusion additive.

[0131] The effect of altering any of the above variables can bedetermined by routine experimentation.

[0132] The following Examples illustrate the principles and practice ofthe present invention. The formulations used to make the compositionsused in the Examples are set out in the Appendix thereto.

EXAMPLE 1 Polyester/Epoxy White Hybrid Coating on a Microwave OvenCavity (MWOC) Production Line

[0133] Two powders were manufactured, Powder S1A and Powder S1B. Theingredients for extrusion (the same standard system in each case,disclosed as Composition S1 in the Appendix) were weighed, dry mixed ina blender and extruded in a twin-screw powder coating extruder.

[0134] The extrudate to form Powder S1A was kibbled to form chips, whichwere then micronised in an impact mill (Hosakawa ACM40) with theaddition of: Aluminium oxide 0.06%

[0135] Extrudate to form Powder SIB was kibbled to form chips. Thesewere blended in a 30-minute tumbling operation with: Aluminiumhydroxide:aluminium oxide  0.5% mixture (55:45 by weight)* PTFE-modifiedpolyethylene wax TF1725 (Lubrizol) 0.07%

[0136] The resulting mixture was then micronised in an impact mill(Hosakawa ACM40) to form Powder S1B.

[0137] The particle size distribution of each powder following themicronising operation was: d_((v)99) 130 microns d_((v)50)  45 microns %< 10 microns   7% % < 5 microns 2.5%

[0138] The finished powders S1A and S1B were then tested on a coatingline to coat microwave oven cavities. The line uses 6 robotic spray gunsfor the cavity interiors, and 6 reciprocating guns for the front part(all of type Gema PGC2). The microwave cavities were hung in columns of3 (referred to herein as Top, Middle and Bottom cavity).

[0139] Firstly, the standard powder formulation milled with aluminiumoxide (Powder S1A) was loaded to the system, and the guns set so that,on coating the cavities, no bare metal was visible on any internal area.This dictated a setting on the atomising air control of 2-bar pressure.Measurements of applied film thickness were made in seven definedlocations on each cavity as shown in the side view below (FIG. 1).

[0140] Powder S1A was then cleared from the system completely.

[0141] The powder milled with wax (Powder S1B according to theinvention) was then loaded to the system, and the guns again set toachieve coverage such that no bare metal was revealed. This required alower setting on the atomising air control, 1.4 bar pressure, ascompared with Powder S1A. Measurements of film thickness over the cavitywere made in the same seven defined locations on each cavity.

[0142] The results were collated in terms of average film thicknessacross the seven measurements, and the standard deviation for thosemeasurements. These results are shown in Table 1 below. TABLE 1 AverageFilm Thickness/Standard Deviation (microns) per Microwave Cavity (MWOC).POWDER JIG POSITION AVE/ Top(μm) Middle(μm) Bottom(μm) Powder MWOC AveStd Dev Ave Std Dev Ave Std Dev Air S1A 125.0 28.6 101.6 27.4 97.1 32.32.0 bar S1B 67.7 15.2 79.2 26.4 63.4 20.7 1.4 bar

[0143] It was found that the total powder requirement to achieveacceptable coverage using Powder S1B according to the invention was 35%less than for the comparison Powder S1A.

EXAMPLE 2 Effect of Different Waxes on a Triboelectric Series

[0144] Analogously to the process described in Example 1, a series ofpowders was made from formulations T1 to T9 as set out in the Appendix,and a quantity of Powder S1A was also taken. In order to establish atriboelectric reference series, the resulting ten powders were tested inpairs so that each powder was tested against every other powder.

[0145] For each test, 10 g of each of the two powders was weighed andplaced into a glass jar. A lid was placed on the jar, and it was shakenthoroughly for two minutes to mix the powders. Following a 30-secondrelaxation time, the lid was removed and the resulting mixture sprayedat two charged panels. The panels were suspended from an earthed clampstand by insulated dips. A voltage was applied to each panel by means ofa Brandenburg high-voltage generator, +20 kV to one, −20 kV to theother, with the current set to the minimum value that would sustain thatvoltage. The glass jar was held with its open end directed toward thepanels, and compressed air was sprayed gently into the jar, such thatthe powder was ejected from the jar and towards the panels in acontrolled stream.

[0146] For each applied powder blend the positive and negative panelswere examined to determine (a) if any separation of the components ofthe blend had occurred on application, and (b) in cases wheresegregation had occurred, which powder had deposited predominantly onthe positive panel, and which powder had deposited predominantly on thenegative panel. From these observed pairs, it was possible to concludethat in every case there was a difference in the composition on eachpanel, and that the powder depositing more on the positive panel waslower in the triboelectric series (more negative), whilst the powderdepositing more on the negative panel was higher in the triboelectricseries (more positive). By a series of such observations, it waspossible to construct the triboelectric reference series shown in Table2 below. TABLE 2 Position of Unmodified Powders in a RelativeTriboelectric Series Powder Code Detail T1 Black hybrid T2 Red hybrid T3Green polyester/Primid T4 Blue hybrid T5 Black hybrid T6 Green hybrid T7White hybrid 2 S1A White hybrid T8 Brown hybrid T9 Yellow hybrid

[0147] It can be seen that Powder S1A, which is Composition S1 withAluminium Oxide as the only post-blended additive, lies near to thebottom of this series.

[0148] Composition S1 was then modified according to the invention, byincorporation of post-blended wax additives to produce six furtherpowders as per Table 3 below. In each case, the specified wax additivewas used in conjunction with a 55%:45% by weight blend of aluminiumhydroxide and aluminium oxide. TABLE 3 Post-Extrusion AdditivesMicronised into Composition S1 Powder Inorganic Code additive Wax S1B0.5% of a 0.07% PTFE-modified PE wax 55:45 blend TF1725 (Lubrizol)) S1Cof 0.07% PTFE-modified PE wax Aluminium TF1780 (Lubrizol) S1D Hydroxide:0.07% pure Polyethylene wax Aluminium PE1500F (Lubrizol) S1E Oxide 0.07%pure PTFE wax TF1790 (Lubrizol) S1F 0.07% polyamide wax A1601 (Lubrizol)S1G 0.07% polyamide wax HM1666 (Lubrizol)

[0149] Analogously to the procedure described above, a new triboelectricseries was constructed using these modified powders. It is seen that theposition of Composition S1 in the series varied markedly according tothe nature of the wax additive incorporated during milling, as shown inTable 4 below. TABLE 4 Triboelectric Series Incorporating ModifiedComposition S1 (Powders S1B-S1G) Powder Code Detail (S1B)/(S1C)(S1/TF1725) (S1/TF1780) T1 Black hybrid T2 Red hybrid T3 Greenpolyester/Primid T4 Blue hybrid T5 Black hybrid S1D/S1E (S1/TF1790)(S1/PE1500F) T6 Green hybrid (T7)/(S1F)/(S1A)/(S1G) (White hybrid 2)(S1/A1601) (S1/Al₂O₃) (S1/HM1666) T8 Brown hybrid T9 Yellow hybrid

[0150] It was not possible by this technique to differentiate betweenwhite powders if they were adjacent to each other in the series, hencein the Table these appear grouped together. The incorporation of waxeshas moved the position of the S1 white hybrid powders in thetriboelectric series vis-a-vis the coloured powders, but a differenttechnique is required, as illustrated in Example 3 below, to distinguishbetween the individual white powders.

EXAMPLE 3 Distinguishing Between White Powders

[0151] In the series of powders from Example 2, there were twounmodified white hybrid formulations, T7 and S1A. White hybrid 2 (T7)was re-made in an identical fashion, except that 0.3% of a commercialred dyestuff (Savinyl Red, ex. Clariant) was incorporated pre-extrusion.This new powder was labelled T10.

[0152] In the triboelectric series established as described in Example2, T7 was placed between T6 (green hybrid) and T8 (brown hybrid).

[0153] Following the same procedure as Example 2, tribostatic testingwas carried out between T6/T10, and T8/T10.

[0154] The inclusion of the low level of the red dye was found to havehad no effect on triboelectric position. T10 was situated below T6 andabove T8, in exactly the same position as the original formulation T7,as illustrated in FIG. 2 below.

[0155] A green version of Composition S1 (white hybrid 1) was then madeby incorporating 0.4% of a commercially available dyestuff (SavinylGreen, ex. Clariant) into the pre-extrusion blend. This was labelledComposition S2 and the formulation is disclosed in the Appendix.

[0156] The colour of T10 was compared spectrophotometrically to itswhite counterpart, T7. This involved establishment of the CIE L*a*b*₁₉₇₆CO-ordinates following measurement on a Datacolor colour managementsystem.

[0157] The parameters used for all measurements in this Example were:Illuminant D65; Observer 10°, Geometry d/8°. These terms will beunderstood by all involved in the measurement of colour, for example inthe textile and coatings industries.

[0158] A procedure developed in accordance with the invention for thedetermination of τ is as set out below. The procedure is applicablegenerally in the practice of the invention and is not restricted to thespecific mixtures described herein.

[0159] Prepare a basic formulation as a chip form (sample A)

[0160] Prepare a basic formulation including a small amount of dyestuff,but otherwise identical to the sample A formulation (sample B)

[0161] Micronise the samples A and B independently to produce powdercoating samples A′ and B′.

[0162] Prepare a 50:50 mixture of A′ and B′ and fluidise/spray atcharged plates to ensure that there is no separation due to theinclusion of the dyestuff (i.e. equal colours on both the positive andnegative plates, or in τ terms, τ_(A′-B′)=0).

[0163] Mix a PTFE-modified wax into chip A at 0.2% and micronise to makea powder C′ (the 0.2% because this powder will subsequently be mixed50:50 with another powder, giving 0.1% wax in the finished powder).

[0164] Mix powder C′ (wax treated, non-dyed) with B′ (non wax treated,dyed) in 50:50 weight ratio.

[0165] Carry out a fluidisation/spraying test using the mixture C′-B′and determine if there is any preferential deposition on the chargedplates. Any colour difference relating to the pure colours of B′ (dyed)and C′ (non-dyed) will enable calculation of τ, in accordance with therelationship given, hereinbefore.

[0166] The fluidise/spray step of the above procedure is as follows:

[0167] Each mixture is charged into a fluidised bed (ITW Gema Volstatic,fluidising air pressure 1 bar) and allowed to fluidise for 30 minutes.The powder is then spray applied using an ITW Gema Volstatic coronaapplication gun with the gun voltage at zero (gun settings: fluidisingair pressure 1.0 bar, conveying air 0.6 bar, supplementary air 3.5 m³hour⁻¹, single point corona needle at zero volts, baffle nozzle). Thesprayed powder cloud is directed towards two panels, one held at −20 kVand another one held at +20 kV. The panel voltage was supplied by meansof two Brandenburg Alpha III high voltage power supply units with thecurrent set to the minimum value that would sustain the voltage.Following application of the powder cloud to the panels for 10 seconds,spraying was stopped, the voltage was removed from the panels, and thecoated panels were stoved (10 minutes at 1 80° C.) in order to fix theapplied powder to the panels for subsequent inspection and analysis.

[0168] The triboelectric interaction factor τ defined hereinbefore wasthen determined for the mixtures, as described above, by measuring thecolour difference between the powders deposited from each mixture ontotwo oppositely charged panels. Since the changes under considerationwere in redness or greenness only, Δa=ΔE and accordingly only Δa wasused.

[0169] Since the powders were to be tested in pairs to determine thedegree of separation between positive and negative panels, thedifference between the pure colours was first measured to establish amaximum as a base line. These values are shown in Table 5, and representa τ of 1.0. TABLE 5 Colour Difference between Pure Red or Green andWhite. Powder 1 Powder 2 Colour Difference Δa S1A (white hybrid, Al₂O₃) S2 (green S1) 25.1  T7 (white hybrid 2, SiO₂) T10 (red T7) 27.7

[0170] This concept is represented pictorially in FIG. 3.

[0171] By reference to these colour differences, it was then possiblefor all future tests to show a τ value for the powder mixture bymeasuring the Δa and expressing it as a ratio of the maximum Δa fromTable 5. If two powders were completely separated in the triboelectricseries, there would be pure colour on each panel, so Δa (max)=Δa(measured). As τ=Δa (measured)/Δa (max), this means τ=1.0. If there wereno splitting, the powders would apply equally to both positive andnegative panels. The Δa would be 0, and Δa(max)/Δa (measured)=0,therefore τ=0.

[0172] Several powder pairs were tested, and the results in terms of τfrom comparing the positive and negative panels are shown in Table 6.TABLE 6 τ values for White Powder Mixtures Powder 1 Powder 2 Δa τ S1 +amide wax T10 21.9 0.79 Ceridust 3910 post- blended S1A S2 + TF1780 PTFE15.9 0.63 wax T7 S2 + A1601 amide 16.5 0.66 wax T10 S1B (S1 + TF172517.9 0.65 PTFE wax)

[0173] On the basis of visual observation of the coatings deposited frompowder mixtures onto positive and negative panels as hereinbeforeexplained, it was possible to establish a triboelectric series for thevarious white hybrids as shown in Table 7. TABLE 7 Triboelectric Seriesof White Hybrids S1B S1 + TF1725 PTFE wax S1C S1 + TF1780 PTFE wax S1DS1 + PE1500F PE wax + S1F S1 + A1601 amide wax T7 White hybrid 2 (nowax) S1A S1 no wax S1G S1 + HM1666 amide wax

[0174] It was thus proven that not only can coloured powders be rankedin the triboelectric series, but also white powders. Furthermore, thestrength of the effect of any particular wax on the triboelectricproperties of a white powder can be expressed in terms of a τ value forthe same base powder with and without the wax addition.

EXAMPLE 4 Effect of Wax in Conjunction with Post-Blended InorganicAdditive

[0175] Powder coating compositions V1-V3 (shown in the Appendix) weremanufactured by weighing out, dry-mixing, and extruding in a twin-screwextruder with a barrel temperature of 110° C. The resulting extrudatewas cooled and kibbled to produce small chips, and blended with thevarious post-blended additives as shown in the Appendix and summarisedbelow: Composition Post-blended additive V1 Fumed silica V2 Aluminiumhydroxide/aluminium oxide blend V3 Wax + aluminium hydroxide/aluminiumoxide blend

[0176] The chip/additive blends were micronised using an Alpine 100 UPZimpact mill and passed through a 150-micron mesh sieve to yield thefollowing particle size distribution: d_((v)99) 130 microns d_((v)50) 55 microns % < 10 microns   7% % < 5 microns 2.5%

[0177] Each powder was tested using a standard set of conditions, spraycoating test pieces as shown in FIG. 4 using an ITW Gema Volstaticcorona application gun, using the procedure outlined below to ensureconsistent application for each powder coating.

[0178]FIG. 4 shows a perspective view of a comer test piece as used inExample 4. Each test piece is formed from three planar sections at rightangles to each other.

[0179] In each test, a comer piece as shown in FIG. 4 was suspended inan application booth from the hole shown in the top of the piece. Thecorner piece was allowed to come to rest at its natural centre ofgravity, as shown in FIG. 5 below, which shows the test arrangement indiagrammatic form

[0180] The powder coating application gun was clamped into position suchthat the gun was pointing directly at the corner of the test piece, witha gun-tip to corner distance of 30 cm. The panel was coated (gunsettings: fluidising air 1.0 bar, conveying air 0.6 bar, supplementaryair 3.5 m³ hour⁻¹, single corona needle conical baffle nozzle at 50 kV).The weight of the coated panel was recorded, and compared with theuncoated weight of the panel. A range of application trials were carriedout for each powder coating composition until an applied weight of 4.0grams of powder coating had been achieved. The resulting coated testpiece was then stoved to give a cured film (stoving conditions: 10minutes at 180° C.), and reserved for further inspection.

[0181] For each coated test piece, the degree of penetration of thecomposition into the comer region was visually assessed.

[0182] In order to remove the subjective nature of an individual visualassessment of the penetration, six people individually assessed thecoated test pieces and ranked the penetration from best to worst leadingto overall assessments as follows: Best<−−−−−−−−−−−−−−−−>Worst V3 V2 V1

[0183] There were very significant differences in the performance of thethree compositions. Thus the worst sample, composition V1, was uncoatedto an average distance of 1 cm. either side of the comer. The best, V3(incorporating both a wax and an aluminium hydroxide/aluminium oxideblend as post-blended additives) was fully coated over the entiresurface.

EXAMPLE 5 Effect of Particle Size Distribution

[0184] Composition S1 was manufactured by dry blending the ingredientsin a shear mixer, extruding in a twin-screw extruder at 110° C., andcooling and kibbling the resultant extrudate to form chips.

[0185] The resultant chip was split into three parts for micronising. Toeach was added Aluminium hydroxide:aluminium oxide mixture (55:45)* 0.5% PTFE-modified polyethylene wax TF1725 (Lubrizol) 0.07%

[0186] Each chip was micronised in an Alpine 100 UPZ impact mill, usingdifferent settings to produce three powders of different particle sizedistribution—Powders S1H, S1I and S1J. The particle size distributionswere as detailed in Table 8 below. TABLE 8 d_((v)99) D_((v)50) Powdermicrons microns % < 10u % < 5u S1H 105.6 33.1 10.2 3.6 S1I 68.9 26.612.7 4.3 S1J 57.8 20.8 16.7 6.1

[0187] The powders were all tested in the same way. The powder wasplaced in a fluidised bed at an air pressure of 1.0 bar, and sprayedinto a microwave cavity from a gun (Gema PGC2) perpendicular to the gunopening, at a distance of 10 cm. from the front face.

[0188] The gun settings were: Conveying air: 1 bar Supplementary air 3m³/hr Rinsing air: 2 m³/hr

[0189] These settings gave a powder output of 150-170 g/min.

[0190] The results were calculated in two ways. Film thicknesses weremeasured at the locations shown in Example 1, and a standard deviationof the measurements was calculated for each powder. Also, a ratio ofpowder thickness in the microwave turntable to the thickness in the backcomer was calculated. The ideal would be a 50:50 ratio. The results forthe three powders are given in Table 9. TABLE 9 Powder Uniformity vs.Particle Size Powder Standard Deviation (um) Turntable:Corner ratio S1H31.2 66.0:34.0 S1I 32.1 67.6:32.4 S1J 35.1 68.2:31.8

[0191] There is a small but significant particle size effect, with thecoarsest powder (S1H) showing the best performance.

EXAMPLE 6 Putting Two Opposite Waxes in Same Powder

[0192] Samples were taken of Powders S1B and S1G from Example 2(Composition S1 with the following post-blended additives) Powder S1BAluminium hydroxide:aluminium oxide mixture (55:45)*  0.5% PTFE-modifiedpolyethylene wax TF1725 (Lubrizol) 0.07%

[0193] Powder S1G Aluminium hydroxide:aluminium oxide mixture (55:45)* 0.5% Polyamide wax HM1666 (Lubrizol) 0.07%

[0194] As a result of the different waxes used, powders S1B and S1G wereat opposite ends of the triboelectric series established in Example 2.The two powders, each incorporating a different wax, were mixed togetherin a 50:50 ratio to produce Powder S1K.

[0195] A powder S1L was then prepared, having the same composition asS1K, by incorporating both waxes (and the aluminium hydroxide/aluminiumoxide blend) with kibbled chip of composition S1 and then milling theresulting blend to the same particle size distribution as Powder S1K.

[0196] In a standard test procedure for coating microwave over cavitiesas outlined in Example 5, it was found that Powder S1K gave asignificantly more uniform coating than Powder S1L, as demonstrated by alower standard deviation between the various measurement locations and aturntable:corner ratio reduced from 3:1 to 2:1.

[0197] These results indicate that, if two different waxes are used, theperformance of the powder will be better if the waxes are each milledseparately with a quantity of the composition, followed by blending ofthe resulting powders, than if both waxes are incorporated togetherprior to milling.

[0198] In general, for the wax-containing compositions of the inventiondisclosed in the foregoing Examples, the tribo-electric interactionfactor τ will be at least 0.5.

1. A powder coating composition which incorporates a wax in post-blendedform.
 2. A powder coating composition as claimed in claim 1, whichincorporates, as further post-blended additives, a combination ofaluminium oxide and aluminium hydroxide.
 3. A powder coating compositionas claimed in claim 1 or claim 2, characterised by separation, in atriboelectric reference series established as hereinbefore defined, ofthe composition incorporating the wax and the same composition withoutthe wax.
 4. A powder coating composition as claimed in claim 3, whereinthere is a wide separation in the triboelectric reference series betweenthe composition incorporating the wax and the same composition withoutthe wax.
 5. A powder coating composition as claimed in claim 1 or claim2, characterised by a triboelectric interaction factor τ as hereinbeforedefined, between the composition incorporating the wax and the samecomposition without the wax, of ≧0.25, ≧0.3, ≧0.4, ≧0.5, ≧0.6, ≧0.7 or≧0.8.
 6. A powder coating composition as claimed in claim 1 or claim 2,characterised by a triboelectric interaction factor τ, between thecomposition incorporating the wax and the same composition without thewax, of ≧0.25, ≧0.3, ≧0.4, ≧0.5, ≧0.6, ≧0.7 or ≧0.8, the value of τbeing given by the relationship τ=ΔE(composition mixture)/ΔE(purecompositions) where ΔE=(ΔL* ² +Δa* ² +Δb* ²)1/2 with L*, a* and b* beingrespectively the z-, x- and y- coordinate variables under the CIEL*a*b*₁₉₇₆ colour definition system, ΔE (pure compositions) beingdetermined by colour spectrophotometric measurement and AE (compositionmixture) being determined by mixing the two compositions in equal weightproportions, causing charging of the resulting mixture by tribostaticinteraction to establish equilibrium tribostatically charged conditions,directing the charged mixture onto two oppositely charged plates,resulting in a separation of the compositions on the two plates, andthen determining ΔE, by colour spectrophotometric measurement, betweenthe compositions as applied to the two plates, one or both of therespective initial pure compositions being dyed where appropriate toprovide an enhanced ΔE between them to facilitate the determination ofΔE (pure compositions) and ΔE (composition mixture).
 7. A powder coatingcomposition as claimed in any one of claims 1 to 6, wherein the wax isselected from the group consisting of polyethylene (PE) wax,polytetrafluoroethylene (PTFE) wax, PE wax modified with PTFE orpolyamide, and polyamide wax.
 8. A process for forming a coating on asubstrate, in which a composition as claimed in any one of claims 1 to 7is applied to the substrate by a powder coating process resulting inparticles of the composition adhering to the substrate, and forming theadherent particles into a continuous coating.
 9. A process as clamed inclaim 8, wherein the powder coating process is a corona applicationprocess.
 10. A process as claimed in claim 8 or claim 9, wherein thesubstrate is an article having recessed portions subject to the Faradaycage effect.
 11. A process as claimed in claim 10, wherein the articlehas multiple faces and the ratio of the minimum to maximum coatingthickness is at least 40%, preferably at least 50%.